In an as-hot rolled microalloyed steel, optimum strength and toughness are conferred by a fine-grained polygonal ferrite structure. Additional strengthening is available via precipitation hardening and ferrite work hardening, although these can be detrimental to the fracture properties. The development of a suitable fine-grained structure by thermomechanical processing or working such as hot rolling can be considered to occur in three or sometimes four stages or regions. In the first, a fine-grained structure is produced by repeated recrystallization of austenite at high temperatures. This is followed, in the second, by austenite pancaking at intermediate temperatures. The third stage involves working the steel at the still lower temperatures of the intercritical region, i.e. the ferrite/austenite two-phase range. Sometimes, further working below the ferrite/austenite two-phase temperature range can occur. For a given chemistry (alloy composition), the final microstructure is dictated by the amounts of strain applied in each of these temperature ranges and the cooling applied after it leaves the rolling mill.
The first stage occurs at temperatures above temperature T.sub.nr, being the temperature below which there is little or no austenite recrystallization. The second stage occurs at temperatures below temperature T.sub.nr but above the temperature Ar.sub.3, being the upper temperature limit below which austenite begins to transform into ferrite. The third stage occurs at temperatures below temperature Ar.sub.3 but above the temperature Ar.sub.1, being the lower temperature limit below which the austenite-to-polygonal ferrite transformation is complete. The final stage occurs below temperature Ar.sub.1. (The designations Ar.sub.3 and Ar.sub.1 are conventionally used to identify the upper and lower temperature limit respectively of the ferrite/austenite two-phase region, as it exists during cooling.) Since only limited improvement in steel quality normally occurs below temperature Ar.sub.1, steel is frequently not rolled below this temperature, although in some cases further such rolling is desirable to further harden the steel albeit at the expense of ductility.
An objective for obtaining superior strength and toughness of steel is to obtain as much fine-grained bainite as possible in the final product. To this end, a specific amount of reduction should occur above the minimum recrystallization temperature T.sub.nr.
In-line controlled cooling apparatus is previously known for use in rolling mills in which steel progresses in-line from a caster through a series of reduction stands and eventually is reduced to a finished product thickness, cut to length and offloaded. At an appropriate stage downstream of the reduction roll stands, controlled cooling apparatus may be provided that imparts to the rolled steel a relatively rapid cooling intended to consolidate the grain structure that has been obtained during the preceding sequence of reductions of the intermediate steel sheet product. The purpose of the controlled cooling is to cool the rolled intermediate product quickly while still fully austenitic, and more importantly, to promote transformation of austenite to bainite, which possesses attractive combinations of strength and toughness.
A problem with this conventional technology is that the steel undergoing the series of reductions is continuously losing heat and dropping in temperature. Because reduction of the steel, while the temperature of the steel remains above the T.sub.nr (the temperature above which recrystallization will occur) imparts fine grain structure to the steel and because the sheet is constantly dropping in temperature, it is desirable to run the steel as rapidly as possible through the series of reduction stands in order to optimize the amount of reduction that can occur above the T.sub.nr. However, such rapid passage of the steel through the series of reduction stands can have at least some undesirable offsetting counter-effects, including:
1. the absence of sufficient time between sequential passes for the desired amount of recrystallization to occur; and PA1 2. the increased capital expenditure required to provide equipment compatible with high-speed rolling mill operation.
Suitable controlled cooling equipment may comprise water spray devices or laminar flow cooling or a combination of both. While in some situations, an immersion cooling might be appropriate, it is seldom suitable for the production of fine-grain bainite steels that is the objective of the controlled-cooling technology heretofore practised.
A further limitation of a conventional rolling line is that the flow-through capacity is limited by the item of in-line equipment having the smallest flow-through capacity. This is true also of Steckel mill lines, an example of one such being disclosed in U.S. Pat. No. 5,414,923 (Thomas et al.). Such Steckel mill lines typically comprise in downstream sequence: a reheat furnace, a Steckel mill with associated coiler furnaces, a downcoiler (or upcoiler), a cooling station, and a plate table with a shear. Of these items of apparatus, typically the maximum-weight capacity of the downcoiler or coiler furnace is substantially less than other items of apparatus in the line. Therefore, the flow-through capacity of some or most of the items of apparatus is not fully utilized; overall production is limited by one of the items of coiling equipment.